In manufacturing semiconductor devices such as LSI and super-LSI or in manufacturing a liquid crystal display board or the like, a pattern is made by irradiating light to a semiconductor wafer or an original plate for liquid crystal, but if a dust gets to adhere to a photo mask or a reticle (hereinafter merely referred to as a “mask” for simplicity) during the irradiation operation, the dust absorbs light or refracts it, causing deformation of a transferred pattern, roughened edges or black stains on a base, and leads to a problem of damaged dimensions, poor quality, deformed appearance and the like.
Thus, these works are usually performed in a clean room, but it is still difficult to keep the mask clean all the time. Therefore, a pellicle is attached to a surface of the mask as a dust-fender before photo irradiation is carried out. Under such circumstances, foreign substances do not directly adhere to the surface of the mask but only onto the pellicle membrane, which is sufficiently removed from the mask surface, and thus by setting a photo focus on a lithography pattern on the mask, the foreign substances on the pellicle membrane fail to transfer their shadows on the mask and thus no longer become a concern to the image transfer performance.
In general, a pellicle is built up of a pellicle frame, which is an endless frame bar, and a transparent membrane or pellicle film, the latter being tensely pasted to one of two frame faces. The membrane material is selected from cellulose nitrate, cellulose acetate, fluorine-containing polymer and the like, which transmits light (g-ray, i-ray, KrF excimer lasers, ArF excimer lasers, etc.) well, and the pellicle frame is made of aluminum, stainless, polyethylene and the like. A solvent capable of dissolving the pellicle film is applied to one of two frame faces of the pellicle frame and the pellicle film is laid onto it and the solvent is air-dried to complete the adhesion, or an adhesive such as acrylic resin, epoxy resin, fluorine-containing resin or the like is used to adhere the pellicle film onto the frame face (hereinafter this face is called “upper frame face”). The other frame face (hereinafter called “lower frame face”) of the pellicle frame is laid with a pressure-sensitive adhesive layer made of polybutene resin, polyvinyl acetate resin, acrylic resin and the like for attaching the pellicle frame to a mask, and over the pressure-sensitive adhesive layer is laid a separation layer (or releaser) for protecting the pressure-sensitive adhesive layer.
The pellicle is installed in a manner such that the pellicle frame encompasses a pattern region extending on the surface of the mask. To accomplish the purpose of the pellicle which is to prevent particles from getting to the surface of the mask, the pellicle is disposed to isolate the pattern region from the external atmosphere so that particles in the external atmosphere are unable to reach the pattern region.
In recent years, the design rules for LSI have shifted toward more fineness to an order of sub-quarter micron, and this has urged shortening of the wavelengths of light sources with the result that mercury lamps for the g-line (436 nm) and i-line (365 nm) are being replaced by KrF excimer lasers (248 nm), ArF excimer lasers (193 nm) and the like. As required fineness is further increased, the flatness required of masks and silicon wafers is increased too.
The pellicle is attached to the mask for the purpose of fending particles off the pattern region only after the preparation of the mask is completed. It happens occasionally that when the pellicle is attached to the mask, the flatness of the mask is changed. If the mask flatness is lowered, problems such as de-focusing may take place. Also, when the mask flatness is lowered, the pattern configuration laid on the mask deviates from the original configuration, and a correct alignment is not obtained when masks are put assembled.
There are a number of causes that lead to change in the flatness of the mask, and it has been found that the most influential one is the comparatively poor flatness of the pellicle frame.
In recent years, the requirement for higher flatness of the mask has been strengthened from a flatness of 2 micrometers in the pattern region face, and since the time 65 nm node was introduced, a flatness of 0.5 micrometer or lower, or preferably 0.25 micrometer is required of the pattern face.